1. Field of the Invention
The present invention relates to a magnetic resonance imaging method for obtaining a magnetic resonance signal (to be referred to as an MR signal hereinafter) of an object to be examined (usually a patient) and an apparatus for the same and, more particularly, to a method and apparatus for decreasing an image quality degradation, caused by an artifact generated in an magnetic resonance image (to be referred to as an MR image hereinafter) due to the presence of a fluid in the object to be examined, e.g., a blood flow and a cerebrospinal fluid (CSF).
2. Description of the Related Art
The assignee of the present invention has already filed in Japan a patent application of a magnetic resonance imaging technique for obtaining a good MR image, which is the precedent of the present invention. This patent application is known as Published Unexamined Japanese Patent Application No. 63-046146. The magnetic resonance imaging technique in this patent application discloses a local excitation method. According to this local excitation method, first, regions adjacent to a prospective data acquisition region are non-signaled so that no MR signal is generated there, and thereafter the data acquisition region is locally excited. In the local excitation method, a high-frequency pulse (to be referred to as an RF pulse hereinafter) having a flip angle of 90.degree. and gradient magnetic fields GX, GY, and GZ as slicing magnetic fields are sequentially applied to an object to be examined placed in the static magnetic field, and the object is sliced in three directions. A three-dimensional local region is excited by slicing. Gradient magnetic fields GX, GY, and GZ having a high magnetic field intensity are applied immediately after excitation of the local region in order to disperse the lateral magnetization components outside the local region. The regions outside the local region are non-signaled by these steps. Then, a desired region (slice plane) in the local region is selectively excited, and an MR signal is acquired from the selectively excited slice plane.
In such a local excitation method, when a T1 intensity image of a component having a short longitudinal relaxation time T1 is to be obtained, magnetization restarts at the initial stage of the gradient magnetic field application. Then, a non-signaling degree outside the local region is partially impaired. As a result, it is difficult to obtain a good image. If a fluid such as a flowing blood and a cerebrospinal fluid is present in the data acquisition region (slice plane), a phase difference occurs to cause an artifact in the MR image.
A clinical inconvenience caused by such problems will be described with reference to FIG. 1. An abdominal tomographic image of an object to be examined is displayed on a monitor screen 10. A liver 12, a heart 14, and a lung 16 appear on this tomographic image. Blood vessels such as an artery and a vein exist in these organs. A belt-like artifact 18 may be generated in the central portion of the screen 10 due to the blood flowing in such vessels. The belt-like artifact 18 may overlap an end portion 12A of the liver 12. As a result, an observer (doctor) may erroneously recognize the end portion 12A of the liver 12 as an abnormal portion even if it is not actually abnormal. Also, if an artifact 18 appears to overlap the end portion 12A of the liver 12 despite that no artifact is generated actually, the doctor may erroneously recognize that the end portion 12A of the liver 12 as a normal portion. This makes it difficult to discover a liver disease such as a liver cancer and a hepatocirrhosis at an early stage.